1. Field of the Invention
The present invention relates to complementary metal-oxide-semiconductor (CMOS) image sensors, and more particularly, to a blue photodiode of a CMOS image sensor and a method for manufacturing the same, in which the blue photodiode is formed to be higher and wider than a semiconductor substrate to improve sensitivity of blue light.
2. Discussion of the Related Art
An image sensor is a semiconductor device that converts optical images to electrical signals. The image sensor is classified into a charge-coupled device (CCD) and a CMOS image sensor. The CCD stores charge carriers in MOS capacitors and transfers the charge carriers to the MOS capacitors. The MOS capacitors are approximate to one another. The CMOS image sensor employs a switching mode that sequentially detects outputs of unit pixels using MOS transistors by forming the MOS transistors to correspond to the number of the unit pixels using CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits.
The CMOS image sensor that converts data of an object into electrical signals includes signal processing chips having photodiodes. Each of the signal processing chips includes an amplifier, an analog-to-digital converter, an internal voltage generator, a timing generator, and a digital logic. The CMOS image sensor is economical in view of space, power consumption, and cost. The manufacture of the CCD requires technical process steps. However, a CMOS image sensor can be manufactured in mass production by a simple etching process of a silicon wafer. Thus, it is cheaper to manufacture a CMOS image sensor than to manufacture the CCD. Also, an advantage of the CMOS image sensor is its packing density.
To display images, the CMOS image sensor sequentially detects signals in a switching mode by forming a photodiode and a transistor in a unit pixel. Also, since the CMOS image sensor uses CMOS technology, low power consumption is required. The number of masks for a CMOS image sensor is fewer by twenty fewer than the thirty to forty masks required for the CCD image sensor. In this way, in the CMOS image sensor, process steps are simplified and various signal processing circuits can be integrated in one chip. Therefore, the CMOS image sensor has received much attention as an image sensor for the next generation.
FIGS. 1 and 2 show a unit circuit of a related art CMOS image sensor.
As shown in FIG. 1, a photodiode PD for generating optical charges using received light, a transfer transistor 101 for transferring the optical charges generated by the photodiode to a floating diffusion region 102 by applying a signal Tx to its gate, a reset transistor 103 for resetting the floating diffusion region 102 by applying a signal Rx to its gate to set the potential of the floating diffusion region 102 at a desired value and for discharging the floating diffusion region 102, a drive transistor 104 serving as a source-follower buffer amplifier by applying a signal Dx to its gate, a selection transistor 105 for addressing and applying a signal Sx to its gate, and a load transistor 106 for providing an output signal (Vb) to be read out from the unit pixel. Power (VDD) is applied.
As shown in FIG. 2, a P-type epitaxial layer 111 is grown on a heavily doped P-type substrate 110, and a lightly doped N-type photodiode region 113, a P-type well 114 and a device isolation film (FOX) 112 are formed in the epitaxial layer 111. The transfer transistor 101 and the reset transistor 103 are formed on the epitaxial layer 111 between the photodiode region 113 and the P-type well 114. The drive transistor 104 and the selection transistor 105 are formed on the P-type well 114. The floating diffusion region 102 is formed on the epitaxial layer 111 between the transfer transistor 101 and the reset transistor 103. A P-type diffusion layer 115 is formed in the lightly doped N-type photodiode region 113 below the surface of the lightly doped P-type epitaxial layer 111 and is doped more lightly than the epitaxial layer.
Referring to FIG. 3, illustrating a blue photodiode portion of the above CMOS image sensor, the device isolation film 112 is formed on the semiconductor substrate 110 in which the lightly doped P-type epitaxial layer 111 is formed. The device isolation film 112 serves to isolate an active region including the lightly doped N-type photodiode region 113. The transfer transistor 101 including a gate electrode is formed on the epitaxial layer 111. A first interlayer dielectric film 116 is formed on the epitaxial layer 111 including the transfer transistor 101. The first interlayer dielectric film 116 is selectively etched to form a via hole 117. Then, a first metal layer (not shown) is deposited and selectively etched to form a first metal layer pattern 118. A second interlayer dielectric film 119 is formed on the first interlayer dielectric film 116 including the first metal layer pattern 118. A second metal layer (not shown) is formed on the second interlayer dielectric film 119 and then selectively etched to form a second metal layer pattern 120. A third interlayer dielectric film 121 including the second metal layer pattern 120 is formed on the second interlayer dielectric film 119. A device passivation layer 122 is formed on the third interlayer dielectric film 121. A blue color filter array element 123 is formed on the device passivation layer 122 as part of a color filter array. The color filter array includes a red filter pattern, a blue filter pattern, and a green filter pattern. A planarization layer 124 is formed on the blue color filter array element 123. Finally, a microlens 125 is formed on the planarization layer 124 and corresponds to the blue color filter array element 123.
In the related art CMOS image sensor manufactured above, however, the blue photodiode may fail to sense shorter wavelengths because blue light in a general pixel structure has a penetration depth of 0.3 μm. By contrast, red light has a penetration depth of 10 μm. Therefore, the desired display of the respective colors (i.e., red, green, and blue) at a ratio of 1:1 is impeded, thereby degrading color reproduction quality.